Understanding the structure and properties of geological formations can improve the efficiency of oil field operations such as drilling, well completion, and production. The collection of information relating to conditions downhole, commonly referred to as “logging,” can employ a wide range of downhole instruments including, for example, wireline and logging while drilling (“LWD”) nuclear magnetic resonance (“NMR”) logging tools.
NMR tools employ a static magnetic field, B0, to give nuclei with non-zero nuclear spin (non-zero magnetic moment and angular momentum) split energy levels. Since lower energy levels are preferred, an ensemble of nuclei will exhibit an anisotropic distribution of energy states, giving the nuclear spins a preferential polarization parallel to the imposed field. This state creates a net magnetic moment and produces a bulk magnetization. The application of this static magnetic field causes the nuclei to converge upon their equilibrium alignment (i.e., to polarize) with a characteristic exponential relaxation time constant. When this convergence occurs after the nuclei have been placed in a cooperative initial state (discussed below), it is known as recovery. The time constant for recovery is called the “spin-lattice” or “longitudinal” relaxation time T1.
During or after the polarization period, the NMR tool applies a perturbing field, usually in the form of a radio frequency electromagnetic pulse whose magnetic component, B1, is perpendicular to the static field B0. This perturbing field moves the orientation of the magnetization into the transverse (perpendicular) plane. Given a static magnetic field strength B0, the frequency of the pulse can be chosen to target specific nuclei (e.g., hydrogen). The polarized nuclei are perturbed simultaneously and, when the perturbation ends, they precess around the static magnetic field gradually re-polarizing to align with the static field once again while losing coherence in the transverse plane (T2 relaxation). Much of their coherence can be restored using a 180° inversion pulse (aka refocusing pulse) at some delay τ/2 after the initial perturbation pulse, leading the precessing nuclei, after another τ/2 delay, to generate a detectable radio frequency “echo” signal having an amplitude that depends on the interecho time τ and, where a sequence of refocusing pulses is employed, on the total time t since the initial perturbation pulse. With one or more sequences of refocusing pulses, the amplitude dependences of the echoes on interecho time τ and total time t can be mapped out to enable measurement of the statistical distributions of T1 and/or T2, and based thereon, measurements of porosities, and/or diffusion constants.
The static magnetic field provided by existing downhole NMR logging tools varies as a function of position, yielding the desired static field strength B0 for a given perturbation signal frequency within a relatively well defined measurement region. Unfortunately, existing tools tend to have perturbation pulses with signal sideband energy that extends over a range of frequencies, so that the well-defined measurement region is not the sole source of the echo signal responses. In the past this issue has been ignored or partially addressed by minimizing the signal sideband energy as much as possible within the limits imposed by the design of the tool. While minimizing sideband energy is helpful for achieving desired accuracy for a particular measurement region, existing schemes do not effectively enable comparison of different measurement regions without undesirable delay between the measurements.
It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.